PART $2$: Avogadro$$s Constant from a Copper Electrolytic Cell

First Electrode Second Electrode

Mass of electrode, initial: Mass of electrode, final: Mass change, absolute value: Moles Cu$,$ absolute value:

For the following calculations, refer to the example on page $12$:$4.$

Current Time Charge $\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_$

How many electrons were transferred? Show the calculation below. Charge on an electron is required. Step $3$

How many copper atoms were etched or plated? Show the calculation below. Need Cu:e$$ mole ratio. Step $4$

Calculate Avogadro$$s constant twice, once using the moles of copper etched and once using the mole of copper plated. Show the calculation below. Step $5$ and Step $6$

Pick the value that is closest to the accepted value of Avogadro$$s number, NA$,$ and calculate the percent error below.PART $2$ Avogadro$$s Constant from a Copper Electrolytic Cell

In this experiment electric current will be used to cause copper to be etched off of one electrode

and deposited onto another. From the change in mass, the number of coulombs transferred and the

molar mass of copper, Avogadro$$s constant will be determined.

The half$-$cell reactions are

Anode: Cu$($s$)->$ Cu$2+($aq$)+2$e$$

Cathode: Cu$2+($aq$)+2$e$->$ Cu$($s$)$

You will notice that there is no net reaction; everything cancels! However, mass will be transferred

from the anode to the cathode. In fact, this is how electroplating is performed.

Supply List

Netbook computer with LoggerPro running

LabQuest Mini interface and the constant current system

Two copper strips $($anode and cathode$)$; copper plating solution

Magnetic stirrer and stir bar; $150-$mL beaker

As the name suggests, the constant current system $($CCS$)$ will attempt to deliver a constant flow

of electric current by automatically adjusting the voltage. The dial on the CCS controls the

current and the computer software merely monitors the actual current delivered. The computer

does not control the device. The crocodile clips on the CCS are difficult to use. So instead attach

double ended alligator clips to them and use these to make connections to the copper strips.

Work in groups of four students for Part $2.$

PROCEDURE

$1.$ Obtain two strips of copper. Clean the surface gently with an abrasive pad only if they look

badly corroded or are flaking. Make sure the electrodes are dry and weigh each of them,

recording their masses on the Data and Analysis sheets.

$2.$ Pour around $100$ mL of Part $2$ copper sulfate solution into a $150-$mL beaker. Place the

copper electrodes into the solution and secure them to the beaker with tape or clothes pins

so that they will not touch each other. Both electrodes must be well submerged into the

solution. Add a magnetic stirring bar and gently stir the solution.

$3.$ Make sure that the LabQuest Mini is connected the computer and that the constant current

system is connected to the Mini and plugged into an outlet. LoggerPro should recognize

that the constant current system is connected.

General Chemistry II Laboratory, Experiment $12$

Electrochemistry: Voltaic and Electrolytic Cells

$12$:$8$

$4.$ Connect ONLY ONE of the alligator clips to one of the copper strips. Turn the constant

current knob to the two o$$clock position. See the diagram below.

$5.$ Click Data Collection $($clock icon$)$ in LoggerPro. Make sure that you are in Time Mode

and that Duration is set for $1800$ seconds. Click the Triggering tab, check triggering and

have it set to turn on at $0\mathrm{.}25$ amperes $($increasing$).$ Click Done.

$6.$ Click the green Collect button in LoggerPro $($erase any previous files, if prompted$),$ then

make the final electrical connection to the other copper strip. The timer will then

automatically start and the current should flow at around $0\mathrm{.}4$ amperes.

$7.$ Watch the time closely near the end. At $1800$ seconds disconnect the clips and remove

the copper electrodes from the beaker. The current will not automatically stop flowing at

$1800$ seconds $$ you MUST disconnect a wire in order to stop the current. Gently rinse the

copper electrodes under some running tap water and then swish them around in the bottle

of acetone $($front of the classroom$)$ to remove surface water. Let them air dry for a couple

of minutes. Do not wipe them dry.

$8.$ Weigh the electrodes and record their masses.

$9.$ Return the Part $2$ copper sulfate solution to the original bottle and the copper electrodes

to their container when you are done. Clean up your work space and unplug the constant

current system.

$10.$ Complete the calculations on the Data and Analysis sheets in order to come up with the

calculated value for Avogadro$$s constant.

Figure $3$ The Electrolytic Cell Setup

wire with alligator clips

dial set to two o$$cloc